Anson explained it somewhere.. 02-05's the 1:1.1 mates up... So if you get an 06, you have to get an 02-05 rear diff.1:1.1 center diff mates up to the WRX front and rear diffs
I think that's it anyway - honestly, I don't know. I got lucky, I bought my 2006 STI trans from someone who had already performed the swap into their 2003. All the work was already done regarding figuring out what was needed. I just bought his old setup and had it put it in my 2002.